Feat: expose file's sequence number for interface data_files_of_snapshot

[chenzl25]

Currently, data_files_of_snapshot returns a vector of DataFile which lacks sequence number information, so it can't be used to support MOR table scan. Any plan for a better interface?

pub async fn data_files_of_snapshot(
        &self,
        snapshot: &Snapshot,
    ) -> Result<Vec<types::DataFile>>